In electrostatographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface forms an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be discharged in a variety of ways.
It is common practice in electrophotography to use corona charging devices to provide electrostatic fields driving various machine operations. Thus, corona charging devices are used to deposit charge on the charge retentive surface prior to exposure to light, to implement toner transfer from the charge retentive surface to the substrate, to neutralize charge on the substrate for removal from the charge retentive surface, and to clean the charge retentive surface after toner has been transferred to the substrate. These corona charging devices normally incorporate at least one coronode held at a high voltage to generate ions or charging current to charge a surface closely adjacent to the device to a uniform voltage potential, and may contain screens and other auxiliary coronodes to regulate the charging current or control the uniformity of charge deposited. A common configuration for corotron corona charging devices is to provide a thin wire coronode tightly suspended between two insulating end blocks which support the coronode in charging position with respect to the photoreceptor and also serve to support connections to the high voltage source required to drive the coronode to corona producing conditions. The coronode is partially enclosed by a conductive shield held at ground potential which serves to increase corona current produced. It is often desirable to have two coronodes within the same structure, which effectively increases the width of the charging zone for the improvement of charging uniformity, and which may be provided by a single wire of double the required length, having free ends of the wire anchored at one end block and a looped end anchored at the opposite end block. It is common in wire coronode structures to provide a spring connector to anchor an end of the coronode to an insulating end block. Scorotron corona charging devices have a similar structure, but are characterized by a conductive screen or grid interposed between the coronode and the photoreceptor surface, and held at a voltage corresponding to the desired charge on the photoreceptor surface. The screen tends to share the corona current with the photoreceptor surface. As the voltage on the photoreceptor surface increases towards the voltage level of the screen, corona current flow to the screen is increased, until all the corona current flows to the screen and no further charging of the photoreceptor takes place. It is to be noted that it is desirable that the screen be supported in a rigid, flat manner, so that it is uniformly spaced from the photoreceptor.
In use, wire coronode corotrons and scorotrons are noted for the ability to produce a reasonably uniform charge on a charge retentive surface. However, over time, the environment to which the coronode is exposed begins to cause irregularities and degradation in charging uniformity. These irregularities may be traced to surface irregularities on the coronode surface which over time becomes pitted, or coated with toner or fuser release agent or other process by-products which must be removed. While cleaning the coronode serves to improve the charging characteristics, coronodes eventually require replacement due to further degradation in performance, or breakage which often occurs while cleaning.
Heretofore, stringing the coronode into position between the insulating end blocks has required a degree of expertise outside the ability of most users. The primary difficulty in the procedure is to obtain the appropriate tension on the wire. If the tension is too low, undesirable sagging of the wire occurs, with resultant charging non-uniformity. If the tension is too high, breakage of the coronode may result. It would be highly desirable to have a corona charging device with a coronode allowing stringing in a simple manner so that a moderately trained user, rather than a service representative, could change or replace the coronode. Such an arrangement would allow significant savings in service time, machine dcown time and allow operator participation in maintaining copy quality. Of course, an added advantage would be the ease of coronode replacement by the service representative, decreasing his time in performing this job on a regular service call.
In the past a wide variety of arrangements have been used to support coronodes in position with respect to the charging device, and maintain a satisfactory degree of tension on the coronode. Thus, for example, U.S. Pat. No. 3,499,143 to Martin discloses a corona charging device including a spool of wire supported within the device so that it is readily available for use. The spool is supported so that it is selectably rotatable to remove wire from the spool for stringing the device, or to hold the end of the coronode in a fixed position. At an opposite end of the corona charging device, a pulley is provided around which the wire is supported and returned to the spool end of the device, where the free end of the wire is anchorable with a screw. A spring biased lever rotates the spool unitl the wire is in a taut condition, prior to fixing the free end at the screw anchor. This arrangement requires significant judgment in the installation of the wire, observing the wire and spring tension. U.S. Pat. No. 4,110,811 to Hubble, III et al discloses a useful arrangement which removes most judgment from the installation, providing a compression spring fixed to a coronode wire at a first end and a fixed ball terminator at a second end, and relies on the compression spring to maintain tension. Such a termination arrangement is relatively expensive, however, and the dielectric materials commonly used for the terminations cannot withstand long exposures to high voltages normally found in corona devices. U.S. Pat. No. 4,258,258 to Laing et al shows a generally similar arrangement in which one end of a coronode is provided with a bead termination and supported within a channel, while the other end extends through an insert to a tensioning means, where cooperating collars serve to retain the coronode end in the insert against any force pulling it out of the insert. Inserts supporting the coronode in position can be made chemically non-reactive. U.S. Pat. No. 3,566,223 to Salger teaches an adjustable wedge-type clamping mechanism. One end of the corona charging device includes a double clamping arrangement to fix both ends of the wire in position, while the opposite end provides a spring loaded abutment, wherein tension on the wire is achieved by removal of the spring.